130 LEECHES 



At metamorphosis the larval ectoderm together with some under- 

 lying muscular and connective tissues and the protonephridia 

 degenerate and are replaced by tissues derived from the germinal 

 bands. The adult gnathobdellid is thus formed almost entirely 

 from germinal band and mesodermal tissue. In the larval stage 

 the bands are divided on each side into an anterior cephalic portion 

 and a posterior trunk portion (Fig. 81). The latter gives rise to 

 the ventral nerve cord in the normal manner but the brain is 

 formed from the cephalic portion of the germinal bands and not 

 from the larval head ectoderm as in the glossiphoniids. 



The endoderm, at first represented by a number of large discrete 

 cells, soon takes on the form of a syncytial gut epithelium by the 

 fusion of the cells and the subdivision of the nuclei. It does not 

 become divided into separate epithelial cells until a iew days after 

 the leeches have left the cocoon, and up to this time the gut lumen 

 is filled with albuminous fluid. The formation of the proctodaeum 

 takes place after the shedding of the larval organs. The passive 

 macromeres A, B and C come to lie between the gut wall and the 

 ectoderm in the posterior part of the embryo, where they gradually 

 degenerate and are absorbed. 



Schoumkine (1953) has shown that in all important respects the 

 embryology of Hirudo corresponds with that of Erpohdella. 



7. Special Features of Piscicolid Embryology 



Eggs of piscicolid leeches cleave normally until after the 

 formation of the second quartette of micromeres but subsequent 

 development is complicated by precocious gastrulation. The events 

 which occur during development are basically similar to those 

 occurring in other groups but the timing is quite different. At the 

 stage when two quartettes have been formed all micromeres except 

 2d begin to multiply and their descendants spread rapidly over the 

 other cells providing a complete covering of very flat ectodermal 

 cells (Fig. 82). Within this outer covering the macromeres and 2d 

 continue to divide, 2d giving rise to the mother cells of a pair of 

 germinal bands and 4d forming a pair of mesoblasts. As the 

 original egg contained very little yolk the macromeres are quite 

 small. They organize themselves into a solid mass, multiply and 

 then form a hollow organ which is the gut rudiment (Fig. 83). At 



